In fluoroscopy-assisted, image-guided surgery, it is necessary to register each C-arm image before it can be used for navigation purposes. To this end, a device (registration kit) which can be localized in a medical tracking system is conventionally attached to the image intensifier of the C-arm. The registration kit has a pattern of radio-opaque markers which are imaged on the C-arm image when the image is produced. The opaque markers have to be arranged in such a way that they do not completely lie in one plane, which means that they have to be provided in a three-dimensional distribution. The relative position of the imaged markers can then be used to determine the centre of projection of the image produced. To this end, the tracked device—i.e. the registration kit—has to have a spatial extension and remain on the C-arm for each image produced, because the centre of projection has a different position relative to the image intensifier for each image-generating direction. This is due to the fact that the C-arm only has a finite rigidity and is subject to different deformations in different positions.
U.S. Pat. Nos. 6,851,855 B2, 6,932,506 B2 and 7,251,522 B2 describe calibration processes for a C-arm, wherein it is always necessary during calibration to always also produce respective calibration images for each image-generating direction. In other words: at each angle for which the C-arm is to be calibrated during calibration, it is also necessary to produce a calibration image in order to ascertain the respective centre of projection.